1. Field of the Invention
The invention relates to thermoplastic molding compositions containing a thermotropic liquid crystal (crystalline) polymer, polycarbonate and polyester. The polymeric composition exhibits improved ductility and toughness properties, such as impact strength and elongation at break over conventional LCP blends. These compositions also show normal mold shrinkages.
2. Discussion of the Background
In general, the properties of a polymeric molding composition depend on the properties of the specific polymers contained in the composition. Many known polymeric compositions have been formulated to take advantage of specific properties and combinations of properties which are achieved by combining together specific polymeric materials.
Fibers are conventionally added to polymer composites to provide fiber-reinforced products. Conventionally, glass fibers, carbon fibers and high-strength organic polymer fibers, such as aromatic polyamide fibers, have been added to polymer compositions to provide improved impact strength and tensile strength. More recently, liquid crystal polymers (LCPs) have been added to polymeric molding compositions to improve the flexural strength of polymer compositions. Conventionally, the components of polymer composition including the LCP are mixed and compounded together followed by extrusion.
Polymer blends containing wholly aromatic liquid crystalline polyesters and a second polymer forming a binary blend are well known in the art. The use of aromatic polyesters which are capable of forming an anisotropic melt phase produces injection molded articles which exhibit good tensile strength, tensile modulus, flexural strength and flexural modulus which exceeds that of the non-LCP polymeric components considered separately.
Examples include U.S. Pat. No. 4,267,289 which discloses binary blends of two aromatic polyesters forming an anisotropic melt phase. U.S. Pat. No. 4,276,397 and U.S. Pat. No. 4,460,736 teach binary mixtures of a polyarylene sulfide with a polyester forming an anisotropic melt phase. U.S. Pat. No. 4,489,190 and U.S. Pat. No. 4,408,022 disclose binary polymer blends containing a polyalkylene terephthalate with polyesters. U.S. Pat. No. 4,460,738 and EP 0 265 791 teach blends of polycarbonate with polyester. U.S. Pat. No. 4,567,227 teaches blends of poly(ester-amide) with polyester and U.S. Pat. No. 4,871,817 discloses blends of poly(ether-imide) with LCPs, including polyesters. U.S. Pat. No. 4,970,285 and U.S. Pat. No. 4,833,229 teach thermotropic copolyester LCPs and their binary mixtures with polycarbonate, PET or PBT. U.S 4,841,009 discloses substantially linear monomeric compositions and liquid crystal polymer compositions.
EP 0 587 732 discloses polymer molding compositions which are fiber-reinforced with reinforcing fibers of a liquid crystalline polymer in a matrix of polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polycarbonate or an unsaturated polyester resin. Additional composites are disclosed in G. Kiss, Polymer Science and Engineering, 27 (6), 410-423 (1987), which teaches blending thermotropic LCPs with amorphous or crystalline polymers. The melt viscosities of these blends are reduced over those of polymers not containing the LCP. The substantial increase in tensile strength demonstrated by these fiber-reinforced blends is offset by a drastic reduction in the elongation at break properties. The addition of increasing amounts of LCP as a reinforcing agent increases tensile and flexural modulus and strength but greatly reduces the elongation at break and embrittles the material.
U.S. Pat. No. 4,439,578 discloses the use of high aspect ratio LCP particles in thermoplastic molding compositions for the purpose of enhancing resistance to melt dripping.
U.S. Pat. No. 4,228,218 discloses the reinforcement of a polymer by incorporating 20% or less of a polymer having a rigid rod shape. The rigid reinforcing polymer is uniformly dispersed in a matrix resin and has a particle size of 50 .ANG. or more. The method of dispersion of the rigid rod polymer in the matrix resin leaves the rigid rod in a state in which it imparts reinforcement to the matrix resin, functioning in a manner similar to reinforcing glass fibers. The reinforced polymers are prepared, for example, by casting films or fibers out of solution. These processes produce shearing forces which cause the rigid rod polymers to line up producing an anisotropic product with the desired reinforcement properties. Since these small rigid rod polymers are not subjected to temperatures and shear rates which would cause them to agglomerate, the rigid rod polymers remain as small rigid rods, acting as an inert filler reinforcing agent. These anisotropic reinforced polymers exhibit good tensile strength, but inadequate ductility and toughness properties.
A ternary polymeric molding composition is described in U.S. Pat. No. 5,011,884. These molding compositions contain 20-70 wt. % of a thermotropic liquid-crystalline polymer, 10-50 wt. % of a polycarbonate and 10-50 wt. % of a polyester. These materials exhibit high flexural strength and a low thermal expansion coefficient, but inadequate ductility and toughness properties. The three components are simultaneously blended to give the desired molding composition.
A need continues to exist for polymeric molding compositions containing liquid crystal polymers having improved properties. In particular, a need exists for LCP containing compositions which have increased toughness and ductility without significant loss of other desirable physical properties such as tensile and flexural modulus and heat deflection temperature.
SUMMARY OF THE INVENTION
One object of the present invention is to provide a polymer blend composition containing liquid crystal polymers which demonstrates improved ductility and toughness properties.
A further object is to provide a liquid crystal polymer containing molding composition which has not only improved toughness properties but also improved tensile and flexural strength, heat deflection temperature and flexural modulus properties over the corresponding binary blends without LCP present.
These and other objects which will become apparent from the following specification have been achieved by the present polymer composition and the method of preparing the same. The polymeric molding composition of the present invention comprises polycarbonate, polyester and thermotropic liquid crystal polymers, further having a unique polymer morphology where the surface morphology of extruded or injection molded solid objects contains substantially no LCP fibrils and where the core of the polymer article contains LCP droplets having a low aspect ratio.
These novel molding compositions are prepared by first blending two of the components together followed by extruding this preblend with the third component, under melt mixing conditions favoring the LCP entering the nematic phase, to form pellets. The melt mixed blend is a ternary blend having improved elongation at break and impact strength.